This invention relates to a machining method performed by a machine tool having axes of rotation for subjecting a workpiece to desired machining by performing the following: moving a tool relative to a workpiece along a cutting path from a predetermined direction to perform machining positioning the tool at a machining starting point on the next cutting path after the completion of machining along the abovementioned cutting path; thereafter moving the tool relative to the workiece along the next cutting path from the predetermined direction to perform machining, and thereafter repeating the foregoing operations. More particularly, the invention relates to a machining tool machining method whereby a path from a machining end point to a machining starting point can be created automatically such that the tool will not be allowed to strike the workpiece when the tool is moved.
In the numerically controlled machining of a curved surface, as shown in FIG. 1, there are cases where a workpiece is subjected to desired machining which includes: moving a tool TL at a cutting velocity from left to right along a predetermined cutting path PTl on a workpiece WK to perform machining; moving the tool in a rapid-traverse mode from an end point Pe to a cutting starting point Ps on the next cutting path PT2 at the conclusion of machining along the abovementioned path; then moving the tool at the cutting velocity along the next cutting path while maintaining the abovementioned cutting direction; and thereafter repeating this unidirectional cutting operation. In such numerically controlled machining of a curved surface, machining is carried out in such a manner that the central axis (the one-dot chain line in FIG. 1) of the tool TL is directed normal to the workpiece WK or oriented in a direction having a fixed angle of inclination with respect to the direction of the normal line at all times. Consequently, by way of example, the machine tool is arranged to rotate the tool while the tool is being moved along orthogonal axes in three dimensions, whereby machining can be performed while the direction of the central axis of the tool is brought into agreement with the direction of the normal line to the workpiece at all times. NC data specifying the tool path includes position data (position vectors) for specifying the position of the tool nose, and tool central axis direction data (positions along B and C axes or tool central axis vector) for specifying the direction of the tool central axis. Note that the B and C axes are vertical and horizontal axes of rotation.
In the machining of a curved surface by such unidirectional cutting, the tool nose will strike the workpiece at high speed when pick-feed is performed, thereby resulting in an erroneous cutting operation or in damage to the tool, unless an appropriate pick-feed path from the first machining end point Pe to the second machining starting point Ps is decided. Previously, when performing pick-feed the pick-feed paths were decided so that the tool nose will not contact the workpiece, and each pick-feed path is programmed as NC data.
However, in the conventional method, a pick-feed path that will not cause a tool to interfere with a workpiece cannot be determined for any and all curved surfaces through a simple technique because each pick-feed path must be individually programmed. As a result, creating the NC data can be a troublesome task. In addition, to assure that the tool will not interfere with the workpiece, with the conventional method the tool retraction stroke is enlarged and, hence, so is the pick-feed stroke. This is disadvantageous in that actual machining time is prolonged.
The foregoing drawbacks become even more pronounced especially when performing pick-feed while rotating the tool in the directions of the B and C axes. The reason is that even when the path of travel of the tool TL of a machine tool having axes of rotation is a straight line LN in three dimensions X, Y and Z, as shown in FIG. 2, the path traversed by the tool nose is unpredictable rather than linear, as indicated by the dashed line in FIG. 1, when the tool is rotated in the directions of the B and C axes at the same time that it is moved along the straight line.